1. Field of the Invention
This invention relates generally to power chain saws and, more particularly, to safety devices for catching and containing a cutting chain that has become detached from the chain bar during operation of the saw. More specifically, the present invention pertains to a device for removing from the chain saw drive sprocket a cutting chain that has become detached from the chain bar during operation of the chain saw portion of a forest harvester to thereby deenergize the cutting chain, contain it, and render it harmless.
2. Background Art
Chain shot—i.e., flying fragments of a broken cutting chain of a power chain saw, as depicted, for example, in FIG. 1—can be caused by improper chain tension, maintenance or repair, a damaged chain drive sprocket, chain guide bar and/or cutting chain, improper chain guide bar and chain lubrication, or excessive chain speed. Chain shot moves at high speed, usually in the plane of the chain bar and forwardly or rearwardly from the saw, and is an ever present danger to nearby personnel during operation of a power chain saw. In the case of a forest harvester, there is a direct hazard to the operator in the harvester cab when the chain bar is pointing toward the cab because chain shot has been known to penetrate the cab window under those circumstances and strike the operator within the cab.
One approach to reducing the hazard presented by a broken cutting chain has been to deenergize and/or impose restrictions on the movement of broken chain ends and chain fragments. U.S. Pat. No. 5,101,567 issued to Cool disclosed a safety switch that deenergized the chain drive of a hand-held chain saw in the event of a break in the cutting chain, and also disclosed a chain braking mechanism and safety guard to effect stoppage of the cutting chain motion. U.S. Pat. No. 4,805,308 issued to Ritola disclosed a roller attachment to the handle of a hand-held chain saw. The roller was aligned with the chain guide bar so as to extend substantially perpendicularly through the plane defined by the chain guide bar in order to intercept the saw chain should it become derailed. U.S. Pat. No. 4,315,370 issued to Horne disclosed a finger piece that acted as a safety surface in the event of breakage of the cutting chain on a hand-held chain saw. The finger piece was located far enough forward from the chain drive to force the whipping length of the chain to pivot and catch the broken chain.
The present invention, however, is primarily directed to improving the safety of chain saws mounted on forest harvesters, which are used in the commercial timber industry for felling and cutting the trunk of trees at suitable intervals. The present invention is particularly directed to deenergizing and restraining the cutting chain of a harvester-mounted chain saw when the cutting chain has become derailed—that is, detached from the chain guide bar and chain drive sprocket of the chain saw; see FIG. 3. In such a harvester chain saw, the cutting chain is looped around a drive sprocket and chain guide bar, similar to a hand-held chain saw, but, unlike a hand-held chain saw, which is typically powered by an internal combustion engine or electric motor, to achieve high-powered, rapid cutting, the drive sprocket is mounted to the output shaft of a hydraulic motor, and the chain saw itself is mounted for pivotal rotation about the motor shaft axis by a hydraulic cylinder within a shielding saw casing or saw box—all under the control of an operator seated within the cab of the harvester. The shielding provided by the saw box is only partial, of course, because the saw box must be sufficiently open to permit sawdust generated during cutting to exit the box, for maintenance access, and for the chain saw to pivot in and out of the box.
For such a harvester-mounted chain saw, U.S. Pat. No. 7,690,407 B2 issued to Annala disclosed a protective wall positioned in the vicinity of the chain drive sprocket that was capable of receiving the movement of the tail of the broken cutting chain with the intent of guiding the broken chain in a desired direction. Annala's protective wall, depicted and labelled as “chain shot guard” in FIG. 2, was perhaps the most common device in use on harvester-mounted chain saws to intercept and restrain a broken cutting chain prior to my present invention. A chain shot guard has commonly been accompanied by a chain catcher, also illustrated in FIG. 2, in the form of a short, cylindrical extension attached to, and coaxially aligned with, the hydraulic motor shaft, and intended to intercept and receive in winding engagement a broken tail portion of a cutting chain. In an alternate version, the chain catcher has been bobbin-shaped, as depicted, for instance, in FIG. 4. In either case, the chain catcher of the prior art extended coaxially from, and rotated with, the hydraulic motor shaft. In extensive testing, I have observed that is counterproductive because, after a cutting chain broke or derailed, at the very time the cutting chain needed to be deenergized, the ongoing rotation of the chain catcher continued to add rotational energy to the broken tail of the cutting chain. In the case of a derailed cutting chain, the usual result was that the cutting chain would break up into three segments, creating chain shot instead of eliminating it.